Process for bleaching webs of fibrous cellulose material with hydrogen peroxide vapor containing water vapor



PROQESS FOR BLEACHING WEBS OF FIBROUS CELLULOSE MATERIAL WITH HYDROGENVAPOR CONTAINING WATER Alfred T. Hawkinson, Hugh G. Hull, and James H.Young, Niagara Falls, N. Y., assignors to E. I. du Pont de Nemours andCompany, Wilmington, Del., a corporation of Delaware No Drawing.Application March 28, 1955 Serial No. 497,432

13 Claims. (Cl. 8-104) This invention relates to an improved bleachingmethod and particularly to a continuous method for bleaching webs offibrous materials with hydrogen peroxide.

Alkaline hydrogen peroxide solutions are widely used for bleachingtextile materials, wood pulps and the like. Such solutions are employedcommercially either by applying the preformed peroxide solution to thematerial to be bleached or by immersing the latter in the solution.Dilute peroxide solutions are generally used for cost reasons. In someso-called saturation methods in which the material to be bleached ismerely dampened with the bleach solution instead of immersed therein,the use of somewhat more concentrated solutions has been proposed.However, rapid and uniform application of bleach solution, whetherconcentrated or not, is difiicult to obtain; consequently, priorsaturation bleaching methods have not been as satisfactory as desired.

From a practical standpoint, it would be highly advantageous to have acontinuous bleaching method which could be integrated with existingcontinuous methods and equipment for forming or processing continuouswebs of fibrous materials such as textiles and paper. In the usual papermachine, a wet pulp sheet containing about 10% pulp (dry basis) iscontinuously formed on a Fourdrinier screen from which it is carried bycontinuous supporting felts between press rolls (usually two sets) whichreduce the water content of the sheet to correspond to a pulp content ofabout 30 to 40%. The sheet is then passed through a dryer consisting ofa large number of heated cylinders or cans over whose surfaces thecontinuously advancing sheet of paper is passed. During such passage,the sheet is pressed against the heated cans by means of a continousfelt, called a dryer felt. The sheet passes through the machine at ratesup to around 1800 feet per minute and higher.

No bleaching method has been available heretofore which could besatisfactorily integrated with continuous paper making operations forsimultaneously, continuously and uniformly bleaching paper webs.Application of a peroxide bleaching solution, e. g., by spraying, to thepaper web ahead of the paper machine dryer to obtain flash bleaching inthe dryer has been proposed but is far from satisfactory because uniformdistribution of the peroxide is dilficult to achieve by spray methods ofapplication in the short time involved due to the high rate of travel ofthe web. The amount of peroxide solution that can be applied by sprayingis limited because the addition of the solution weakens the Web. If thesolution is sprayed on at places where the web is mechanicallysupported, e. g., on the screen, a large portion of the bleach solutionis lost by removal on the screen and in the suction boxes and pressrolls, making such method prohibitively expensive.

It is an object of the invention to provide an improved continuousmethod for bleaching webs of fibrous materials. A further object is toprovide a continuous method for bleaching such webs, wherein an aqueoussolution of a conditioning agent and hydrogen peroxide vapor arecontinuously applied uniformly to the web as it is continuouslyadvanced. A particular object is an improved and highly efficient methodfor bleaching paper webs which can be readily integrated with the usualpaper making operation. Further objects will be apparent from thefollowing description.

The objects of the invention are accomplished by separately andcontinuously applying to a moving web of the fibrous material which isto be bleached (a) a liquid comprising a non-volatile water-solubleconditioning agent, such as a non-volatile alkaline material and/or astabilizer, and (b) hydrogen peroxide vapor, and then subjecting saidweb to conditions under which bleaching will occur. After application ofthe conditioning agent and hydrogen peroxide vapor, the web will mostgenerally be heated to accelerate bleaching. However, heating can beomitted and bleaching can be effected at ordinary temperature over alonger period of time if desired.

Any non-volatile water-soluble conditioning agent which enhances orpromotes peroxide bleaching or functions as a stabilizer for peroxidecan be used. Examples are watersoluble non-volatile alkaline materialssuch as the alkali metal hydroxides and alkaline reacting salts thereof,e. g., caustic soda, sodium carbonate, trisodium phosphate and borax,which function primarily as alkalizing agents or alkaline buffers forpromoting bleaching. Examples of stabilizers, some of which alsofunction as alkalizing agents, are the alkali metal silicates, such assodium silicate, and the molecularly dehydrated phosphates such as thesodium pyrophosphates, sodium tripolyphosphate and sodiumhexametaphosphate; also, magnesium salts such as magnesium sulfate andsilicate. These conditioning agents or mixtures thereof are applied inliquid form, e. g., as aqueous solutions. Application thereof to the webmay be subsequent or prior to the application of the hydrogen peroxidevapor. Application prior thereto is generally preferred.

Application of the liquid comprising the conditioning agent can be madein any desired manner, e. g., by wellknown spray or roller applicationmethods. Use of relatively concentrated solutions of the agent isusually desirable so as to avoid introducing into the web an undulylarge amount of water.

It has been found that hydrogen peroxide vapors are rapidly absorbed bythe moving web, particularly when the latter is damp, so thatapplication can be readily effected simply by contacting the moving webfor a short interval of time with an atmosphere containing hydrogenperoxide vapor. Such contact can be readily effected by passing the webcontinuously through an enclosure containing hydrogen peroxide vapor andto which such vapor is continuously supplied as required. Alternatively,the moving web can itself be made to constitute a wall of such anenclosure whereby one side of the web will be contacted with thehydrogen peroxide vapor-containing atmosphere. It has been found thateven when the vapor is contacted with only one side of the web,absorption is effective. This rapid and effective absorption of theperoxide and the fact that effective amounts of peroxide can be readilyapplied and absorbed without undue dilution by solvent are important andhighly advantageous features of the present method.

An especially effective way of applying the peroxide is by means of aslot-type applicator positioned transversely of the moving web and incontact therewith, the peroxide vapor mixture being continuouslycontacted with the web through the slot. Use of such an applicatorintegrated with a paper machine has indicated that amounts of peroxideefiiective for bleaching purposes can be applied substantiallyquantitatively in contact times of the order of 0.2 to 0.05 second orless when using a mixture of water and hydrogen peroxide vapors. Suchrapid and eificient absorption of hydrogen peroxide is truly remarkableand makes practical its application at web speeds of up to 1800 feet perminute and higher.

Hydrogen peroxide vapor may be produced for present purposes by anyconvenient method, e. g., by passing a stream of an inert carrier gassuch as nitrogen, air or steam, through a solution of hydrogen peroxide.In such method, the hydrogen peroxide vapor content of the resultinggaseous mixture can be controlled as desired by controlling the peroxideconcentration in the solution, the temperature of the solution and therate of gas flow. Another practical way of producing the hydrogenperoxide vapor is by vaporizing a hydrogen peroxide solution by theapplication of heat thereto. The hydrogen peroxide vapor content of thegas mixture used can be varied over a wide range. Absorption of thehydrogen peroxide vapor occurs rapidly and effectively even atconcentrations as low as 1.0 mg. per liter, but in most applicationsconcentrations of at least 0.5 H vapor by volume, preferably 1.0 to 15%H 0 vapor by volume, will be employed. Higher concentrations, e. g., upto approximately 22%, can be used if desired but such higherconcentrations usually will be less practical.

Application of either the conditioning agent or the hydrogen peroxidevapor can be effected at a single application station or at a pluralityof such stations. Also, either can be applied to one or both sides ofthe moving web.

The continuously advancing web to which the above materials are appliedwill preferably be damp at the points of application. The free watercontent of the web will generally be within the range of about to 70%,based upon the total weight of the damp web, a water content rangingfrom about to 70% being preferred. Such water content may be derived bypreviously applying water to the web, e. g., by spray or immersionmethods, or it may represent water originating from a previousprocessing operation, e. g., in the formation of a paper web or sheet ina paper making machine.

A distinct advantage in applying the conditioning agent and hydrogenperoxide vapor to webs containing around 5 to 70% free water is thatsuch webs absorb the applied materials very rapidly. Another advantageis that at such moisture contents absorption of a given amount ofhydrogen peroxide results in a substantially higher peroxideconcentration in the web than can be obtained when the same amount ofperoxide is applied by prior solution application methods. This higherconcentration is important since the higher the peroxide concentrationthe greater will be the driving force for the bleaching reaction.

The amount of hydrogen peroxide to be applied in any instance willdepend upon the amount of bleaching intended. equal to at least 0.01% H0 based upon the dry weight of the fibrous material will be desired.Most generally, the absorbed peroxide will range from about 0.1 to 3.0%.Absorption of such or larger amounts can be readily effected by thevapor application method of this invention. The amount absorbed willdepend upon several factors including the time of contact between theweb and the treating gas mixture and the hydrogen peroxide vapor contentof the latter. In general, longer contact times and higherconcentrations of peroxide vapor in the gas mixture increase the amountof peroxide absorbed. It has been found that absorption of amounts ofperoxide adequate for most bleaching purposes can be readily effected insuch remarkably short contact times as to permit integrating thebleaching operation with the usual paper making operation of a papermachine. Such integration is of great practical value in that bothoperations are effected simultaneously in the same equipment, with onlyminor changes in the paper machine being required to permit separateapplications of the conditioning In most cases, absorption of hydrogenperoxide i agent and hydrogen peroxide vapor to the paper web as itpasses through the machine.

When integrating bleaching with paper making, as indicated, the hydrogenperoxide vapor can be applied effectively at any stage following theformation of the web on the Fourdrinier screen. Application between thepress rolls and the dryer, where the water content of the damp web isgenerally in the range of to will be preferred in some cases, however,application can be made if desired in the dryer, e. g., between dryingcans, or following the dryer. The conditioning agent is preferablyapplied to the formed web in advance of the dryer, most preferably at astation in advance of that at which the hydrogen peroxide is applied.Thus, a solution of the conditioning agent may be applied, e. g., by asprayer or roller applicator, across the width of the moving web betweenthe two sets of press rolls just ahead of the dryer, while the peroxidevapor is conveniently and effectively applied between the last set ofpress rolls and the dryer. Either the conditioning agent or the peroxidevapor can be applied during formation of the web, e. g., on theFourdrinier screen, but such practice is not generally practical becausea substantial part of the materials so applied would be lost with thewater removed on the screen and by the subsequent action of the suctionboxes and the press rolls.

The conditioning agent, generally an alkaline material and/or astabilizer, will be added in amounts intended to provide the desiredpromoting and/ or stabilizing effect in the resulting bleach solution. Amixture of conditioning agents will frequently be desirable. A mixturewhich is usually very effective is a combination of caustic soda and amolecularly dehydrated phosphate, e. g., sodium pyrophosphate, soproportioned and added at such a rate as to result in the web containingabout 0.01 to 0.5% caustic soda and about 0.1 to 1.0% sodiumpyrophosphate based upon the dry weight of the fibers. However, theamounts of such agents used may be varied considerably depending uponthe material being bleached, the amount of peroxide used and thebleaching conditions employed.

The invention is illustrated by the following examples.

Example 1 A newsprint paper furnish consisting of unbleached groundwoodand 15 unbleached sulfite pulp was continuously formed into a web ofpaper on a paper machine operating at a speed of about 175 feet of paperper minute. An aqueous conditioning solution containing caustic soda andsodium pyrophosphate was continuously applied uniformly across the widthof the paper web on the Fourdrinier screen over the third of 4 suctionboxes by means of a roller applicator. The water content of the web atthe point of application was about The solution was applied at such arate that the web as it left the last press roll before the dryercontained about 0.5 sodium pyrophosphate (Na P O- and 0.017% causticsoda (NaOH), based upon the dry weight of the web.

Hydrogen peroxide vapors were applied to the web at a station betweenthe last press roll and the dryer where the water content of the web wasabout 60%. Application was made through two application chambers, one oneach side of the web and so positioned that the advancing webconstituted one side of each chamber. Each of the chambers extendedacross the width of the web and 6 inches along its length. A gas mixturecontaining about 45% H 0 vapors by volume, 45-50% water vapor and thebalance air was supplied continuously to each of the chambers. Thehydrogen peroxide absorbed by the web during the short time of travelbetween the two chambers was 1.1% H 0 based upon the dry weight of theweb.

The water content of the web following application of the peroxide wasabout 60%; after passage through the dryer it was about 7%. Aftercalendering and standing Q one day in roll form, the paper had abrightness of 64.8 units compared with a brightness of 5629 units forsimilar unbleached paper. Brightness values were determined using aGeneral Electric reflectometer.

Experience has shown that the conditioning agent can be more efiicientlyapplied between the press rolls where the water content of the web is7075%.

Example 2 A printing paper furnish consisting of 70% unbleachedgroundwood and 30% unbleached sulfite pulp was formed into paper andsimultaneously bleached substantially as described in Example 1, exceptthat the composition and rate of application of the conditioningsolution were such that caustic soda and sodium pyrophosphate contentsof the Web were 0.5% and 0.033% (dry basis), respectively, and the pointof application was just ahead of the first press roll where the watercontent of the web was about 85%. Also, the amount of absorbed hydrogenperoxide was 1% (dry basis). The bleached roll of paper had a brightnessof 66.8 units compared with 59.2 units for a control roll of unbleachedpaper. Printing properties of the bleached roll were the same as for thecontrol roll.

Example 3 The general procedure of Example 1 was repeated except thatthe furnish used consisted of 70% bleached groundwood and 30% bleachedsulfite pulp; and the conditioning solution contained only sodiumpyrophosphate, and was applied so as to give a sodium pyrophosphatecontent in the web of 0.5% (dry basis). After 8 days standing in theroll, the brightness of the treated paper was 71.7 units compared with63.3 units for the control.

Example 4 The general procedure of Example 1 was repeated except that afurnish of 100% unbleached groundwood pulp was used and the absorbed Hcontent of the web was 0.80.9% (dry basis). The brightness of thetreated paper after 6 days in the roll was 66.7 units compared with 56.9units for the untreated control.

Example The general procedure of Example 1 was repeated except that thefurnish was 100% unbleached sulfite pulp and the absorbed H 0 content ofthe web was 0.75% (dry basis). After 5 days in the roll, the brightnessof the treated paper Was 62.3 units compared with 51.2 units for theuntreated control.

In addition to the paper furnishes indicated in the examples, otherfurnishes composed of various combinations and proportions of the pulpsindicated, as well as other paper or wood pulps which normally respondto peroxide bleaching, can be successfully bleached by the presentmethod. Such other pulps include kraft, soda, semi-chemical, andchemigroundwocd pulps. It is also applicable generally to the bleachingof any fibrous material known to be bleachable by means of hydrogenperoxide in the presence of a non-volatile water-soluble conditioningagent. In addition to paper webs, it is applicable to the bleaching oftextile fabrics composed of animal, vegetable or synthetic fibers.However, since it is especially efiective when employing alkalineconditioning agents, it is most useful for bleaching webs composed ofcellulcsic fibers, e. g. paper Webs and cotton textile materials. Theconditioning agent, the amount thereof used, and the bleachingconditions, e. g. temperature, Will of course be chosen keeping in mindthe particular type of fibrous material being bleached in any giveninstance. Any of the conditioning agents and bleaching conditions whichare effective for a given fibrous material in prior saturation bleachingmethods are generally effective in practicing the present method.

Most generally, the conditioning agent and hydrogen peroxide vapor willbe applied to the web while the latter is at ordinary or slightlyelevated temperature although the web can be at considerably highertemperatures if desired. Following their application, the web can bepermitted to bleach simply by storing the same at ordinary or somewhatelevated temperature for a time sufiicient for the desired bleachingaction to occur. Such storage is preferably carried out under conditionsminimizing vaporization of water from the web, e. g. by storing the webin a rolled-up or stacked mass. Most preferably, the web Will be heatedimmediately after application of the peroxide vapor to acceleratebleaching. In the case of paper webs, heating by passage through thedryer of the paper machine is very effective, in which case heating andevaporation of water from the web occur simultaneously to eflectsubstantially all or a major part of the bleaching in the dryer.However, in some instances, the paper from the dryer will still containa substantial amount of hydrogen peroxide so that further bleachingoccurs during storage of the paper roll from the dryer. Heating can alsobe effected under conditions such that little or no evaporation of waterfrom the web occurs, e. g. by direct steaming. Heating withoutevaporation of water, e. g. by steaming, may be desirable and distinctlypreferred in some instances, e. g. in cotton bleaching, whereevaporation of water may result in damage to the goods.

We claim:

1. A bleaching process comprising separately and continuously applyingto a moving damp web of a fibrous cellulose material bleachable withhydrogen peroxide (a) a liquid, non-volatile, water-soluble conditioningagent effective in enhancing the bleaching action of hydrogen peroxideupon said material, and (b) hydrogen peroxide vapor, said damp webcontaining free water equal to from 5 to 70% of its Weight and saidvapor being applied by contacting the moving damp web with a mixture ofhydrogen peroxide vapor and at least 45% by volume water vapor for atime of from 0.05 to 0.2 second until from 0.01 to 3% hydrogen peroxidebased upon the dry weight of said fibrous material is absorbed by saidweb, and thereafter bleaching said web with the absorbed hydrogenperoxide in the absence of any further amount of hydrogen peroxide.

2. A process for bleaching a fibrous cellulose material bleachable withhydrogen peroxide comprising continuously applying to a continuouslymoving damp web of said material at one application station a liquid,nonvolatile, water-soluble conditioning agent efiective in enhancingthebleaching action of hydrogen peroxide upon said material, said damp webcontaining free water equal to from 5 to 70% of its weight, continuouslyapplying hydrogen peroxide vapor to said continuously moving web atanother application station by contacting a mixture of hydrogen peroxidevapor and at least 45% by volume water vapor with said moving web for atime of from 0.05 to 0.2 second until from 0.01 to 3% hydrogen peroxidebased upon the dry weight of said fibrous material is absorbed, andthereafter bleaching said web with the absorbed hydrogen peroxide in theabsence of any further amount of hydrogen peroxide.

3. The process for bleaching cellulose fibers comprising separately andcontinuously applying to a continuously advancing damp web of cellulosefibers containing free water equal to from 5 to 70% of its weight (a) anaqueous solution of a non-volatile alkaline material effective inenhancing the bleaching action of hydrogen peroxide on said fibers, and(11) hydrogen peroxide vapor, said vapor being applied by contacting themoving web with a mixture of hydrogen peroxide vapor and at least 45 byvolume water vapor for a time of from 0.05 to 0.2 second until from 0.01to 3% hydrogen peroxide based upon the dry weight of said fibers isabsorbed by said web, and thereafter bleanching siad web with theabsorbed hydrogen peroxide in the absence of any further amount ofhydrogen peroxide.

4. The process for bleaching cellulose fibers comprising continuouslyapplying to a continuously advancing damp web of cellulose fibers at afirst application station an aqueous solution of a non-volatile alkalinematerial effective in enhancing the bleaching action of hydrogenperoxide on said fibers, said damp web contain ing free water equal tofrom to 70% of its weight, continuously applying hydrogen peroxide vaporto said continuously advancing web at a later application station bycontacting the moving web with a mixture of hydrogen peroxide vapor andat least 45% by volume water vapor for a time of from 0.05 to 0.2 seconduntil from 0.01 to 3% hydrogen peroxide based upon the dry weight ofsaid fibers is absorbed, and thereafter bleaching said web with theabsorbed hydrogen peroxide in the absence of any further amount ofhydrogen peroxide.

5. The process for bleaching cellulose fibers comprising continuouslyapplying to a continuously advancing damp web of cellulose fibers at afirst application station an aqueous solution of a non-volatile alkalinematerial effective in enhanching the bleaching action of hydrogenperoxide on said fibers, said damp Web containing free water equal tofrom 5 to 70% of its weight, continuously applying hydrogen peroxidevapor to said continuously advancing web at a later application stationby contacting the moving web with a mixture of hydrogen peroxide vaporand at least 45 by volume water vapor for a time of from 0.05 to 0.2second until from 0.01 to 3% hydrogen peroxide based upon the dry weightof said fibers is absorbed, and thereafter heating said web in theabsence of any further amount of hydrogen peroxide as it is continuouslyadvanced.

6. The process of claim 5 wherein the web is composed of paper pulp.

7. The process of claim 6 wherein the solution of a non-volatilealkaline material contains a hydrogen peroxide stabilizer.

8. The process of claim 6 wherein, after application of the hydrogenperoxide vapor, the web is heated and dried as it is continuouslyadvanced.

9. The process of bleaching paper during the continuous formationthereof on a paper making machine comprising continuously applying to apaper web during its continuous advancement through a paper makingmachine an aqueous solution of non-volatile alkaline material effectivein enhancing the bleaching action of hydrogen peroxide on said web, saidapplication being made between the web forming screen and the dryer ofsaid machine, and continuously and separately applying hydrogen peroxidevapor to said web during its advancement through said machine bycontacting the moving web with a mixture of hydrogen peroxide vapor andat least by volume water vapor for a time of from 0.05 to 0.2 seconduntil from 0.01 to 3% hydrogen peroxide based upon the dry weight of theweb is absorbed, and bleaching said web with the absorbed hydrogenperoxide in the absence of any further amount of hydrogen peroxide.

10. The process for bleaching paper during the continuous formationthereof on a paper making machine comprising continuously applying to apaper web during its continuous advancement through a paper makingmachine an aqueous solution containing a non-volatile alkaline materialeffective in enhancing the bleaching action of hydrogen peroxide on saidweb and a hydrogen peroxide stabilizer, said solution being applied tothe web at a first station in the machine between the web forming screenand the dryer, and continuously applying hydrogen peroxide vapor to saidweb in said machine at a second application station by contacting themoving web with a mixture of hydrogen peroxide vapor and at least 45% byvolume water vapor for a time of from 0.05 to 0.2 second until from 0.1to 3% hydrogen peroxide based upon the dry weight of said web isabsorbed, said second station being subsequent to said first station,and bleaching said web with the absorbed hydrogen peroxide in theabsence of any further amount of hydrogen peroxide.

11. The process of claim 10 wherein said second application station isbetween the last press rolls and the dryer.

12. The process of claim 10 wherein the aqueous solution applied at thefirst station contains caustic soda and sodium pyrophosphate.

13. The process of claim 12 wherein the aqueous solution applied at thefirst station contains, from 0.01 to 0.5% caustic soda and 0.1 to 1.0%sodium pyrophosphate based upon the dry weight of the web.

References Cited in the file of this patent UNITED STATES PATENTS1,163,438 Muller Dec. 7, 1915 1,759,341 Baum May 20, 1930 2,150,926Kauffmann Mar. 21, 1939 2,189,378 Kauffmann Feb. 6, 1940 2,433,370Kauffmann Dec. 30, 1947 2,492,047 KBurg Dec. 20, 1949 2,777,749 YoungJan. 15, 1957

1. A BLEACHING PROCESS COMPRISING SEPARATELY AND CONTINUOUSLY APPLYINGTO A MOVING DAMP WEB OF A FIBROUS CELLULOSE MATERIAL BLEACHABLE WITHHYDROGEN PEROXIDE (A) A LIQUID, NON-VOLATILE, WATER-SOLUBLE CONDITIONINGAGENT EFFECTIVE IN ENCHANCING THE BLEACHING ACTION OF HYDROGEN PEROXIDEUPON SAID MATERIAL, AND (B) HYDROGEN PEROXIDE VAPOR, SAID DAMP WEBCONTAINING FREE WATER EQUAL TO FROM 5 TO 70% OF ITS WEIGTH AND SAIDVAPOR BEING APPLIED BY CONTACTING THE MOVING DAMP WEB WITH A MIXTURE OFHYDROGEN PEROXIDE VAPOR AND AT LEAST 45% BY VOLUME WATER VAPOR FOR ATIME OF FROM 0.05 TO 0.2 SECOND UNTIL FROM 0.01 TO 3% HYDROGEN PEROXIDEBASED UPON THE DRY WEIGHT OF SAID FIBROUS MATERIAL IS ABSORBED BY SAIDWEB, AND THEREAFTER BLEACHING SAID WEB WITH THE ABSORBED HYDROGENPEROXIDE IN THE ABSENCE OF ANY FURTHER AMOUNT OF HYDROGEN PEROXIDE.